#WorldRobberFlyDay | Curator of Diptera

OK, I have decided to create #WorldRobberFlyDay. All the time now, we hear that this large mammal or that large mammal has a ‘day’, and that got me thinking. Buglife have an Invertebrate of the Month, but even they are not very often the lesser-known insects, including the flies.

And I wanted global. Let the world celebrate! Why is it always the large stuff or the pretty (and, in my opinion, slightly less important) species? So I thought about it and decided it was about time that we championed more aggressively the rights of the small and endangered flies. These creatures are some of the most charismatic animals on the planet. The robberflies, or Asilidae, are truly worth celebrating for their looks, for their behaviour, for their good deeds to us, and because many of them are threatened.

Figure 1. Phylogenetic arrangement of Diptera showing the more advanced Brachycerans and the position of the Asilidae (robberflies) within it

The UK boasts 28 species of Asilidae (OK, so that’s not a lot in terms of flies, but hold on – we have only 30 native terrestrial mammals, of which 17 are bats and 2 are native marine mammals). Globally there are more than 7,500 species, and as such, it is one of the largest families of insects today. In fact Torsten Dikow, a world expert on this group, has them as the third most speciose group of diptera. This is a group, therefore, that has a large impact on the environment in which they live.

Asilidae are Brachycerans (Fig. 1), which are the more advanced and robust flies. Asilidae are known from the Jurassic era, but some of the more important finds are from the Cretaceous, including those from the Crato Formation of north-eastern Brazil (approximately 112 million years old). This site is truly extraordinary in terms of the invertebrate remains that were found there (and just another reason for me to get back to Brazil!).

It was again Linnaeus, or Linne, who described these flies in his 10th edition (1758) Systema Naturae when he erected the genus Asilus. Within this, eleven species were described and then a further four were added in the 12th edition. You may be unsurprised to know that most of these are no longer in the original genus! Ten have been moved to other genera, three we are unsure of due to the original descriptions being vague, so that leaves only two in the genus.

However, the species Asilus crabroniformis, commonly called the hornet robberfly in the UK – and the type species of the family – still sits within this genus in all its magnificence. The division of flies into different families came later with Latreille, a very eminent entomologist who tried to put some more organisation into the entomological hierarchy in 1802. Since then we have increased the number of species and have split the family into many subfamilies -14 in fact (Fig. 2) But as regular readers know, Dipteran taxonomists are still not satisfied and expect more movement in the future.

Even still, you can comprehend how much work has gone on into understanding the relationships within this family so far.

Moustaches and mouthparts

So why are people interested in these flies? Well once more, this is a family of flies that rock! And these rock harder than most. All armed with moustaches and powerful piercing mouthparts, these predators are aptly named, as they truly are the most vicious and effective aerial predators. These flies are venomous, probably both as adults and as larvae (although we know so very little about the offspring). The adults are able to catch, then sedate, their prey whilst on the wing, suck out the contents and then drop the husk of what was once a living breathing entity. It’s almost poetry.

And to be fair, to catch these little predators you often have to become a predator yourself. There is no majestic leaping around the countryside, freely swinging your nets with wild exuberance: instead you must ‘become the fly’. You stalk it; determine where it rests and then strike. If you are me, this is often followed by a squeal of delight or a wail of despair. I once spent a glorious afternoon on one of the Isles of Scilly at the beach (obviously working very hard) trying to stalk these flies. My volunteer and I tried to work in unison hunting them, and I could almost hear the flies mocking us…

The adults are most active during sunny, hot conditions. Again, another reason for loving flies – they have an affinity for the nicer weather conditions.

Although these flies range a lot in size, from 2mm to 6cm, they all share distinctive features that help identify the family. The adults have enormous eyes, which is one of the many tools that make them such efficient predators. And it also helps us recognise this family easily. The bulbous eyes and the distinct dip between the two eyes are very characteristic (see Fig. 3). They can swivel their heads around and their eyes can see what’s going on behind them as well.

Figure 3. My, my… what big eyes you have!

Some of them scout amongst the grasses, their rapid wingbeat enabling them to turn whilst hovering. These truly are the stealth-bombers of the insect world.

The leptogastriniiae are the skinniest of the Asilidae, with very long bodies and legs. They use these long, gangly first two pairs of legs to catch their prey whilst – we think – using the third pair to stabilise themselves. Not all actively scan like this: some will sit and wait, only darting out to impale their prey when they are ready. If fact, there are several different ways in which they hunt and, as with all good scientists, someone has devised a terminology for all of these (Fig. 4)

Figure 4. Lehr (1979) from the Geller-Grimm Asilidae site

For that is another characteristic of this group – a well-formed, stout beak often hidden in a luxurious moustache or, more correctly termed, a mystax (Fig. 5).

It is the needle-like hypopharynx (Fig. 5) that pierces their prey. This is not for the faint-hearted, as they often try and pierce the soft parts of the insect, such as the neck or sometimes the eyes. They have this moustache (Mystax – Fig. 5) to help protect their mouthparts from the flailing prey.

They don’t have to flail for long, though, as the fly injects saliva that contains nerve toxins that paralyse the prey, and proteolytic enzymes that dissolve the insides. They are nasty for insects, spiders, and occasionally a very unfortunate hummingbird, but apart from giving a nasty jab, they are not dangerous to humans. Research done by Adamovic in 1963 found that injecting robberfly saliva into invertebrates kills them instantly, but they never inject venom into humans. There are several researchers in the Natural History Museum who are now studying the venoms within these flies, so watch out for future Museum publications to follow what is happening in this field.

But this leads me to one of the first reasons that these flies are very important. It’s because they are such good predators. Within the UK, between 1930 and 1933, Hobby produced a list of the prey records (Fig. 6).

Figure 6. Hobbies lists from Stubbs and Drake 2014

We have spent the last century working out the prey species and now have a greater understanding of the potential impact these flies can have in helping control populations of species that we often consider as pests – with aphids being a classic example. Although they are opportunistic species, they can have an impact on the overall densities and therefore become the gardener’s friends.

Flirty flies

So let’s move on to courtship. As with most creatures, some do, some don’t; with some species the males just grab, while others put a fair amount of time and effort into it and have different modifications on their bodies to both attract the opposite sex as well as hold on to them. And it’s not just the males that do the flirting. Oh no – there are some females that entice the male.

The rather unusual courtship of the British robberfly Choerages marginatus was described by Ian Rabarts in 2009 (paraphrased from Alan Stubbs’ rather amusing synopsis on the subject, in his and Martin Drake’s book British Soldierflies and their Allies): Firstly the flies recognise that (a) they are the right species, and (b) that they are of the opposite sex (a very good start in most situations to do with copulation leading to fertilisation).

Then they check out each other’s hunting moves and, if OK, the female stands facing the male in a sort of ‘yeah, you’ll do’ posture. After this, she flies in a slow ‘flaunting’ circuit (hussy) very similar to that of a prey item (all very kinky). He attacks when he sees her ‘shimmer-strip’, whereupon she slows down her flight, but flies in an angular pattern. He realises then that this is his lady and adjusts his attack from one of capturing prey to one of copulation.

Alan then states in his book: ‘Failure [of copulation] results in going back a few steps in the courtship sequence.’ A not-unfamiliar event…

Bob Lavigne, a collaborator of mine and another international robberfly expert, wrote in 2003: ‘It is postulated that courtship first developed when male search flights (which end abruptly with copulation), were consistently unsuccessful.’ It sounds so final when it ends with copulation!

In fact, reading the literature when it comes to robberfly mating in copulation has been very entertaining. Morgan (1995) records that another species that were just about to do the do were scared off by a sheep! Given the size difference I too in a similar position may have been scared off…

But check out Pegesimallus teratodes (Fig.7) – these have amazing structures on their hind legs. These are used in the dance of the males to attract the females -they are indeed the peacocks of the robberfly world.

Figure 7. Pegesimallus teratodes and its amazing legs

And that is not all that is fantastic about the males. I would be remiss if I didn’t mention the genitalia of the males (Fig. 8).

Figure 8. The male Pegesimallus teratodes with his rather impressive genitalia

And then there are specimens in our collection that we think give us an indication of a courtship story, although I doubt we will ever be able to find out for certain. Take, for example, two specimens of Mallophora infernalis from our collection (Fig. 9). Now, had the female caught the bush cricket and the male had thought:”Excellent! Both food and sex!”? Or, had the male caught the cricket to attract the female? Either way, it was not going to end well for the bush cricket (or in this case for the robberflies).

Figure 9. Male and Female of Mallophora infernalis who were caught mid air carrying this bush cricket

So whether there is dancing, waving, differences in wingbeats, or offerings, the end result hopefully is the production of eggs. And blimey, the females have a big range of ovipositors (egg laying tubes) (Fig. 10)!

Figure 10. Ovipositors (adapted from Stubbs and Drake 2014)

Now this is where it gets tricky, as we know less about the egg and larval stage than any of the others. And this is the main reason why we should be concerned about these gorgeous creatures – many of the UK species are rare. We have no real idea for many species globally but can only assume that this is the case everywhere. In fact, several of our UK species are protected.

However we don’t know much, if anything, about many of the species’ diet, where they live, development and so on. In Collins’ book The Conservation of Insects and their Habitats, he discusses how little is known about the species, despite the fact that they are classed as threatened.

Take one of the most charismatic insects in the UK (no bias there) the hornet robberfly Asilus Crabroniformis – a mimic of (you guessed it) a hornet. There is still very little information. Previous work dating back to the 90s states that the eggs were laid in or under the old dung of cows, horses and rabbits, and soil nearby. Maybe the adults (and subsequent larvae) are that flexible in their habitat? The larvae are then thought to feed on dung beetles but again this has only been observed (and not by many authors) during late-stage instars. What do the little ones eat? It is a UK priority species and we need to know more about it. How can we consider conserving a species (if it needs it) if we don’t know where it is or what it’s getting up to? It’s like a wayward teenager.

Now, if you want to know more about what is going on with UK robberflies, there are loads of pages giving you what information there is.

There is a nice little piece by naturespot (Fig. 11) featuring some of the UK species, and of course you must check out the Dipterists Forum for all of their information.

Figure 11. The Naturespot website

But what we really want now is information coming the other way. Personal observations in the field, the location of eggs and the like, and species distributions are all critical in ensuring that we maintain and enhance our existing populations.

Martin Harvey @kitenet runs the UK recording scheme for these wonderful little animals (See Fig. 12 or visit the website) and you can send all your records to that site. Martin also runs many courses on these as do others in the Dipterists Forum, so sign up and go along to them.

Figure 12. The Soldierfly and allies recording scheme, which includes the robberflies

So there you go – robberflies are amazing, and they do need celebrating. And if you still need convincing here is a little fluffy one to tug at your heartstrings. When asked what is my favourite fly, Laphria flava is at the center of my heart (Fig. 13).

Figure 13. Laphria flava male

P.S. We moved to WordPress as a new blogging platform for the Museum during 2015. To see the earlier posts in the Curator of Diptera series, visit our archive.